Telescope Reviews: Dumb question about Hubble artificial star...

SCTs can be problematic to collimate during the daytime using an artificial star.

You can of course collimate it, but when the scope is horizontal, the mirror carrier will settle evenly against the baffle at the front and the rear of its length.

When you point the scope upward though, the mirror will take a different orientation. One side of the mirror will be held up (or settle against) the focuser rod threads, while the other side of the mirror carrier will tilt down to rest against the baffle.

The amount of shift away from perfect collimation will depend on the amount of play between the mirror carrier and baffle and on the thickness of the grease film, but in most cases, given a few minutes, the weight of the mirror will cause it to sink though the grease film.

For this reason, if you want the absolute most perfect collimation (which despite popular belief that it must be exact for the scope to perform decently), you should collimate with the scope pointing at the sky.

You should also always approach final focus by using a counter-clockwise motion of the focuser knob. There are several good reasons to always do this, but from a collimation standpoint, approaching using CCW turns "Lifts" the primary mirror into position so that the focuser screw side comes closer to its final rest position every time.

If you focus using back and forth turns (chase focus), the mirror can continue to move slighty after you release the know as the mirror settles against the baffle.

If the mirror is fairly tight on the baffle you might not have and issue, but for people using SCTs with some mirror mirror shift while focusing, collimation should always be done with the scope pointing up and with the final motion of the focuser knob in a CCW direction to test final collimation.

I have not collimated mY C14, C8, or C5 in years. Lots of time people that are having to tweek collimation are having to do it because they are fighting mirror shift.

I you follow this practice, you elminate a lot of fine collimation fussing.

Also, approaching focus using CCW motions is really the best way for viewing as well.

If you overshoot a tiny bit, most people have enough visual accomdation that their eye will automatically bring the center of the field back into best focus. I always make my final approach using CCW motion (Inward on refractors for the same reason).